Active attenuation of vibrations resulting from firing of acoustic sources

Abstract

An acoustic logging tool is provided with a first acoustic source that generates an acoustic signal primarily directed into the formation, the acoustic signal including a noise signal carried axially along the tool, a second acoustic source adapted to generate a noise canceling signal along the tool that actively and significantly cancels the noise signal, and a plurality of acoustic sensors axially spaced from each other along the tool and spaced from the first acoustic source and the second acoustic source. The acoustic sensors receive and record indications of pressure signals resulting from the acoustic signal and the noise canceling signal. The pressure signal indications may be processed in order to generate information regarding the formation.

What is claimed is: 
     
       1. An acoustic logging tool for use in a borehole traversing a formation, comprising:
 a) a first acoustic source that generates an acoustic signal primarily directed into the formation, said acoustic signal including a noise signal carried axially along the tool; 
 b) a second acoustic source adapted to generate a noise canceling signal along the tool that actively cancels said noise signal; the second acoustic source is located between the first acoustic source and a plurality of acoustic sensors; 
 c) the plurality of acoustic sensors axially spaced from each other along the tool and spaced from said first acoustic source and said second acoustic source, said acoustic sensors receiving and recording indications of pressure signals resulting from said acoustic signal and said noise canceling signal; 
 d) at least one vibration sensor spaced between the first and second acoustic source and the plurality of acoustic sensors, the at least one vibration sensor adapted to measure velocity or acceleration of said noise signal and said noise canceling signal; and using said first acoustic source and said vibration sensor in calibrating said tool in order to determine a form of said noise canceling signal. 
 
     
     
       2. An acoustic logging tool according to  claim 1 , wherein:
 said at least one vibration sensor is located between said second acoustic source and said plurality of acoustic sensors. 
 
     
     
       3. An acoustic logging tool according to  claim 1 , wherein:
 said second acoustic source is adapted to direct said noise canceling signal axially. 
 
     
     
       4. An acoustic logging tool according to  claim 1 , wherein:
 said first acoustic source and said second acoustic source are adapted to substantially simultaneously respectively generate said acoustic signal and said noise canceling signal. 
 
     
     
       5. An acoustic logging tool according to  claim 1 , further comprising:
 a processor unit that generates from said indications of pressure signals an indication of a property of the formation. 
 
     
     
       6. An acoustic logging tool according to  claim 5 , wherein:
 said indication of a property of the formation is an indication of a formation slowness. 
 
     
     
       7. An acoustic logging tool according to  claim 6 , wherein:
 said formation slowness is at least one of rock compressional slowness and rock shear slowness. 
 
     
     
       8. An acoustic logging tool according to  claim 5 , wherein:
 said first acoustic source and said second acoustic source are adapted to respectively generate said acoustic signal and said noise canceling signal at least one millisecond apart such that said plurality of acoustic sensors substantially separately sense and record signals resulting from said acoustic signal and said noise canceling signal, and said processor unit is adapted to add or subtract said signals resulting from said acoustic signal and said noise canceling signal. 
 
     
     
       9. An acoustic logging tool according to  claim 1 , wherein:
 said noise canceling signal generated by said second acoustic source is a selected signal of a plurality of different noise canceling signals generated for different formation slownesses. 
 
     
     
       10. An acoustic logging tool according to  claim 9 , wherein:
 said plurality of different noise canceling signals generated for different formation slownesses includes at least three different noise canceling signals. 
 
     
     
       11. An acoustic logging tool according to  claim 1 , wherein:
 said noise canceling signal generated by said second acoustic source is a selected signal of a plurality of different noise canceling signals generated for different borehole diameters. 
 
     
     
       12. An acoustic logging tool according to  claim 1 , wherein:
 said noise canceling signal generated by said second acoustic source is a selected signal of a plurality of different noise canceling signals generated for use of the acoustic logging tool in boreholes having different mud types. 
 
     
     
       13. An acoustic logging tool for use in a borehole traversing a formation, comprising:
 a) a drill; 
 b) a collar; 
 c) a first acoustic source located on said collar that generates an acoustic signal primarily directed into the formation, said acoustic signal including a noise signal carried axially along the collar; 
 d) a second acoustic source located on said collar adapted to generate a noise canceling signal along the collar that actively cancels said noise signal; the second acoustic source is located between the first acoustic source and a plurality of acoustic sensors; 
 e) the plurality of acoustic sensors axially spaced from each other along the collar and spaced from said first acoustic source and said second acoustic source, said acoustic sensors receiving and recording indications of pressure signals resulting from said acoustic signal and said noise canceling signal; 
 f) at least one vibration sensor spaced from the first and second acoustic source and the plurality of acoustic sensors, the at least one vibration sensor adapted to measure velocity or acceleration of said noise signal and said noise canceling signal; and 
 g) a processor unit that generates from said indications of pressure signals an indication of a property of the formation, wherein said noise canceling signal generated by said second acoustic source is a selected signal of a plurality of different noise canceling signals generated for at least one of (i) different formation slownesses, (ii) different borehole diameters, and (iii) use of the acoustic logging tool in boreholes having different mud types. 
 
     
     
       14. An acoustic logging tool according to  claim 13 , wherein:
 said noise canceling signal is a selected signal of a plurality of different noise canceling signals generated for at least two of said (i) different formation slownesses, (ii) different borehole diameters, and (iii) use of the acoustic logging tool in boreholes having different mud types. 
 
     
     
       15. An acoustic logging tool according to  claim 13 , wherein:
 said first acoustic source and said second acoustic source are adapted to substantially simultaneously respectively generate said acoustic signal and said noise canceling signal. 
 
     
     
       16. An acoustic logging tool according to  claim 13 , wherein:
 said indication of a property of the formation is an indication of a formation slowness. 
 
     
     
       17. A method of investigating an earth formation traversed by a borehole, comprising:
 a) providing a borehole tool including (i) a first acoustic source that generates an acoustic signal primarily directed into the formation, said acoustic signal including a noise signal carried axially along the tool, (ii) a second acoustic source adapted to generate a noise canceling signal along the tool that actively cancels said noise signal; the second acoustic source is located between the first acoustic source and a plurality of acoustic sensors, and (iii) the plurality of acoustic sensors axially spaced from each other along the tool and spaced from said first acoustic source and said second acoustic source; 
 b) firing said first acoustic source thereby generating the acoustic signal and firing said second acoustic source thereby generating the noise canceling signal; 
 c) recording at said plurality of acoustic sensors a plurality of indications of pressure signals resulting from said acoustic signal and said noise canceling signal; 
 d) processing said plurality of indications and providing therefrom an indication of a property of the formation; and 
 e) said borehole tool further includes (iv) at least one vibration sensor spaced between the first and second acoustic source and the plurality of acoustic sensors, the at least one vibration sensor adapted to measure velocity or acceleration of said noise signal and said noise canceling signal and using said first acoustic source and said vibration sensor in calibrating said tool in order to determine a form of said noise canceling signal. 
 
     
     
       18. A method according to  claim 17 , wherein:
 said indication of a property of the formation is an indication of at least one of a formation compressional slowness and formation shear slowness. 
 
     
     
       19. A method according to  claim 17 , wherein:
 said firing comprises firing said first acoustic source and said second acoustic source substantially simultaneously. 
 
     
     
       20. A method according to  claim 18 , wherein:
 said noise canceling signal generated by said second acoustic source is a selected signal of a plurality of different noise canceling signals generated for at least one of (i) different formation slownesses, (ii) different borehole diameters, and (ii) use of the acoustic logging tool in boreholes having different mud types. 
 
     
     
       21. An acoustic logging tool according to  claim 1 , wherein the vibration sensor is an accelerometer.